This invention relates to a pressure sensor assembly including a resilient seal for sealing a pressure vessel opening and retaining the pressure sensor in the opening, and more particularly to a seal that provides reduced and controlled insertion and extraction forces.
Motor vehicle engine controls sometimes require measurement of gas pressures subject to positive pressure fluctuations that tend to expel or blow out the pressure sensor. For example, the gas pressure in the intake manifold of an internal combustion engine experiences a sudden positive surge during an engine backfire. A similar condition can occur with a fuel tank vapor pressure sensor in the event of a crash. Although the usual solution in such applications is to use screw fasteners or the like to secure the pressure sensor to the wall of the pressure vessel, it has been found that the sensor can be adequately secured with significant reductions in material and assembly expense by fitting the stem of the sensor with a resilient, flexible annular seal having a retention flange that is larger than the pressure vessel opening. Such a seal is disclosed in the U.S. Pat. No. 5,317,924 to Maack, issued on Jun. 7, 1994, assigned to the assignee of the present invention, and incorporated herein by reference.
Referring to FIG. 1, the seal 10 disclosed in the aforementioned U.S. Pat. No. 5,317,924 includes a plurality of fins 12 of approximately the same diameter as the opening 14 in a pressure vessel wall 16, a retention range 18 having a diameter that is larger than the opening 14, and a central axial bore 20. The pressure sensor 22 has a ported stem 24 that terminates in an enlarged cylindrical foot 26, and the stem 24 is inserted through the seal bore 20 until the seal 10 is retained between the foot 26 and a housing 28 of the sensor 22. When the assembly is inserted into the opening 14, the retention flange 18 extends through the opening 14 and seats against the interior periphery of the pressure vessel wall 16 about the opening 14. The fins 12 prevent leakage between the seal 10 and the opening 14, and the flange 18 and foot 26 retain the sensor 22 in the opening 14 in the presence of a high positive gas pressure in the pressure vessel. When the sensor 22 must be removed for repair or replacement, it may be extracted with a suitable tool capable of exerting an extraction force sufficient to deform and compress the retention flange 18.
In usage, the insertion and extraction forces are held to reasonably low values by appropriate selection of the seal composition and by coating the seal with an oil-based lubricant prior to insertion of the pressure sensor into the pressure vessel opening. For example, the seal may be a fluorosilicone polymer, and the lubricant may be a silicone emulsion. The fluorosilicone material is very resilient and flexible by nature, and the silicone-based lubricant remains on the seal so that the extraction force is not significantly increased over time. In a typical application of the sensor depicted in FIG. 1, the average insertion force may be approximately seven pounds, while the average extraction force may be approximately eighteen pounds.
While the above-described approach has been successfully used in automotive applications for several years, newly proposed emission regulations severely restrict the use of fluorosilicone materials and silicone emulsion lubricants in engine applications. For example, many manufacturers now specify the use of fluorocarbon instead of fluorosilicone, and only water-based lubricants. Unfortunately, fluorocarbon is less flexible than fluorosilione, and water-based lubricants tend to evaporate over time. As a result, the force required to extract a sensor equipped with a fluorocarbon seal is unacceptably high. Also, it would be desirable from a cost standpoint to eliminate the seal lubricant entirely. Accordingly, what is needed is a seal design that will permit the use of relatively stiff material such as fluorocarbon and no lubricant, and still provide acceptably low insertion and extraction forces.
The present invention provides an improved self-retaining pressure sensor assembly including a pressure sensor having a housing and a depending ported stem terminated in an enlarged foot and a flanged resilient seal disposed about the stem between the pressure sensor housing and foot, wherein the seal flange has one or more notches about its circumferential periphery to ease insertion and extraction of the assembly with respect to a circular opening formed in a pressure vessel. The notches reduce the effective area of the flange, proportionately decreasing the force (insertion force) needed to insert the assembly at the time of installation and the force (extraction force) needed to subsequently remove the assembly for repair or replacement. The number and size of the notches can be adjusted to provide acceptable insertion and extraction forces, without lubricant, while not significantly compromising the self-retention capability of the assembly.